Abstract:

Disclosed herein is a display apparatus, including: a plurality of
subpixels disposed adjacent each other and forming one pixel which forms
a unit for formation of a color image; the plurality of subpixels
including a first subpixel which emits light of the shortest wavelength
and a second subpixel disposed adjacent the first subpixel; the second
subpixel having a light blocking member disposed between the second
subpixel and the first subpixel and having a width greater than a channel
length or a channel width of a transistor which forms the second
subpixel.

Claims:

1. A display apparatus, comprising:a plurality of subpixels disposed
adjacent each other and forming one pixel which forms a unit for
formation of a color image;said plurality of subpixels includinga first
subpixel which emits light of the shortest wavelength anda second
subpixel disposed adjacent said first subpixel;said second subpixel
havinga light blocking member disposed between said second subpixel and
said first subpixel and having a width greater than a channel length or a
channel width of a transistor which forms said second subpixel.

2. The display apparatus according to claim 1, wherein said light blocking
member is disposed in parallel to the longitudinal direction of said
second subpixel.

3. The display apparatus according to claim 1, wherein said light blocking
member is disposed in such a manner as to optically cover the transistor
of said second subpixel against the emitted light of said first subpixel.

4. The display apparatus according to claim 1, wherein said light blocking
member is made of a material same as that of a metal line which forms an
anode electrode of an electro-optical element in said second subpixel.

5. The display apparatus according to claim 1, wherein said light blocking
member is disposed below an auxiliary line wired between said first
subpixel and said second subpixel.

6. The display apparatus according to claim 5, wherein said light blocking
member is made of a material same as that of said auxiliary line.

7. The display apparatus according to claim 1, wherein said first subpixel
has a light blocking member disposed between said first subpixel and said
second subpixel and having a width greater than a channel length or a
channel width of a transistor which forms said first subpixel.

8. The display apparatus according to claim 1, wherein each of said
plurality of subpixels includes a writing transistor for writing an image
signal and a driving transistor for driving an electro-optical element in
response to the image signal written by said writing transistor.

9. The display apparatus according to claim 8, wherein each of said
plurality of subpixels has a function for a mobility correction process
of correcting a mobility of said driving transistor by negatively feeding
back a correction amount corresponding to current flowing to said driving
transistor to a potential difference between a gate and a source of said
driving transistor.

10. The display apparatus according to claim 9, wherein the mobility
correction process is carried out in parallel to the writing process of
the image signal by said writing transistor.

11. The display apparatus according to claim 9, wherein the mobility
correction process is carried out while a source voltage of said driving
transistor is raised.

12. An electronic apparatus, comprising:a display apparatus includinga
plurality of subpixels disposed adjacent each other and forming one pixel
which forms a unit for formation of a color image,said plurality of
subpixels includinga first subpixel which emits light of the shortest
wavelength anda second subpixel disposed adjacent said first
subpixel,said second subpixel havinga light blocking member disposed
between said second subpixel and said first subpixel and having a width
greater than a channel length or a channel width of a transistor which
forms said second subpixel.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]This invention relates to a display apparatus and an electronic
apparatus, and more particularly to a display apparatus of the flat type
or flat panel type wherein a plurality of pixels each including an
electro-optical element are arranged two-dimensionally in rows and
columns, that is, in a matrix, and an electronic apparatus which
incorporates the display apparatus.

[0003]2. Description of the Related Art

[0004]In recent years, in the field of display apparatus which display an
image, a flat type display apparatus wherein a plurality of pixels or
pixel circuits each including a light emitting element are arranged in
rows and columns, has been popularized rapidly. One of such flat type
display apparatus uses, as a light emitting element of a pixel, an
electro-optical element of the current driven type whose emitted light
luminance varies in response to the value of current flowing through the
element. As the electro-optical element of the current driven type, an
organic EL (Electro Luminescence) element is known which utilizes a
phenomenon that an organic thin film emits light when an electric field
is applied thereto.

[0005]An organic EL display apparatus which uses an organic EL element as
an electro-optical element of a pixel has the following characteristics.
In particular, the organic EL element has a low-power consumption
characteristic because it can be driven by an application voltage equal
to or lower than 10 V. Since the organic EL element is a self luminous
element, the organic EL display apparatus displays an image of high
visibility in comparison with a liquid crystal display apparatus which
displays an image by controlling the intensity of light from a light
source using liquid crystal for each pixel. Besides, since the organic EL
element does not require a light source such as a backlight, it
facilitates reduction in weight and thickness of the organic EL display
apparatus. Further, since the speed of response is as high as
approximately several μsec, an after-image upon dynamic picture
display does not appear.

[0006]The organic EL display apparatus can adopt a simple or passive
matrix type or an active matrix type as a driving method therefor
similarly to the liquid crystal display apparatus. However, although the
display apparatus of the simple matrix type is simple in structure, it
has a drawback in that it is difficult to implement the same as a
large-sized high definition display apparatus because the light emitting
period of each electro-optical element decreases as the number of
scanning lines, that is, the number of pixels, increases.

[0007]Therefore, in recent years, development of an active matrix display
apparatus wherein the current to flow through an electro-optical element
is controlled by an active element provided in a pixel in which the
electro-optical element is provided such as, an insulated gate type field
effect transistor has been and is being carried out vigorously. As the
insulated gate type field effect transistor, a thin film transistor (TFT)
is used popularly. The active matrix display apparatus can be easily
implemented as a large-sized and high definition display apparatus
because the electro-optical element continues to emit light over a period
of one frame.

[0008]Incidentally, it is generally known that the I-V characteristic,
that is, the current-voltage characteristic, of the organic EL element
suffers from deterioration as time passes as known as aged deterioration.
In a pixel circuit which uses a TFT particularly of the N channel type as
a transistor for driving the organic EL element by current (such
transistor is hereinafter referred to as driving transistor), if the I-V
characteristic of the organic EL element suffers from aged deterioration,
then the gate-source voltage Vgs of the driving transistor varies. As a
result, the luminance of emitted light of the organic EL element varies.
This arises from the fact that the organic EL element is connected to the
source electrode side of the driving transistor.

[0009]This is described more particularly. The source potential of the
driving transistor depends upon the operating point of the driving
transistor and the organic EL element. If the I-V characteristic of the
organic EL element deteriorates, then the operating point of the driving
transistor and the organic EL element varies. Therefore, even if the same
voltage is applied to the gate electrode of the driving transistor, the
source potential of the driving transistor changes. Consequently, the
source-gate voltage Vgs of the driving transistor varies and the value of
current flowing to the driving transistor changes. As a result, since
also the value of current flowing to the organic EL element varies, the
emitted light luminance of the organic EL element varies.

[0010]Further, particularly in a pixel circuit which uses a
polycrystalline silicon TFT, in addition to the aged deterioration of the
I-V characteristic of the organic EL element, a transistor characteristic
of the driving transistor varies as time passes or a transistor
characteristic differs among different pixels due to a dispersion in the
fabrication process. In other words, a transistor characteristic of the
driving transistor disperses among individual pixels. The transistor
characteristic may be a threshold voltage Vth of the driving transistor,
the mobility μ of a semiconductor thin film which forms the channel of
the driving transistor (such mobility μ is hereinafter referred to
simply as "mobility μ of the driving transistor") or some other
characteristic.

[0011]Where a transistor characteristic of the driving transistor differs
among different pixels, since this gives rise to a dispersion of the
value of current flowing to the driving transistor among the pixels, even
if the same voltage is applied to the gate electrode of the driving
transistor among the pixels, a dispersion appears in the emitted light
luminance of the organic EL element among the pixels. As a result, the
uniformity of the screen image is damaged.

[0012]Therefore, various correction or compensation functions are provided
to a pixel circuit in order to keep the emitted light luminance of the
organic EL element fixed without being influenced by aged deterioration
of the I-V characteristic of the organic EL element or aged deterioration
of a transistor characteristic of the driving transistor as disclosed,
for example, in Japanese Patent Laid-Open No. 2007-310311.

[0013]The correction functions may include a compensation function for a
variation of the I-V characteristic variation of the organic EL element,
a correction function against the variation of the threshold voltage Vth
of the driving transistor, a correction function against the variation of
the mobility μ of the driving transistor and some other function. In
the description given below, the correction against the variation of the
threshold voltage Vth of the driving transistor is referred to as
"threshold value correction," and the correction against the mobility
μ of the driving transistor is referred to as "mobility correction."

[0014]Where each pixel circuit is provided with various correction
functions in this manner, the emitted light luminance of the organic EL
element can be kept fixed without being influenced by aged deterioration
of the I-V characteristic of the organic EL element or aged deterioration
of a transistor characteristic of the driving transistor. As a result,
the display quality of the organic EL display apparatus can be improved.

SUMMARY OF THE INVENTION

[0015]Incidentally, if light having high energy is inputted to the channel
of a transistor in a pixel in a state wherein a certain fixed voltage is
applied to the transistor in the pixel, then the threshold voltage of the
transistor shifts to the negative side. In particular, if blue light
having a relatively short wavelength and hence having high energy is
inputted to the transistor, then the characteristic shift of the
transistors becomes very great in comparison with that when no light is
inputted as seen from FIG. 26.

[0016]As an example, subpixels where subpixel units each including three
subpixels for R (red), G (green) and B (blue) are disposed such that the
B subpixel is positioned at the center in each unit are considered. The B
subpixel is influenced by blue light only when the B subpixel itself
emits light.

[0017]However, since the R and G subpixels are positioned adjacent the B
subpixel, even if they themselves do not emit light, they are influenced
by light emitted from the B subpixel positioned adjacent thereto. Where
the R and G subpixels are influenced not only by light emitted from them
themselves but also by light emitted from the adjacent subpixel, it is
very difficult to compensate for a current variation in a correction
process or a like process.

[0018]Although the foregoing description is given in regard to a B
subpixel in color coating of RGB, this similarly applies also to a
subpixel which emits light of the shortest wavelength and hence of the
highest energy in any other color coating.

[0019]Accordingly, it is demanded to provide a display apparatus and an
electronic apparatus wherein it is possible to suppress a characteristic
shift of a transistor of a subpixel which appears when light having high
energy is inputted to the channel of the transistor.

[0020]According to an embodiment of the present invention, there is
provided a display apparatus including a plurality of subpixels disposed
adjacent each other and forming one pixel which forms a unit for
formation of a color image, the plurality of subpixels including a first
subpixel which emits light of the shortest wavelength and a second
subpixel disposed adjacent the first subpixel, the second subpixel having
a light blocking member disposed between the second subpixel and the
first subpixel and having a width greater than a channel length or a
channel width of a transistor which forms the second subpixel.

[0021]Emitted light having a relatively short wavelength has high energy.
In the display apparatus, if light having the shortest wavelength and
having high intensity is inputted from the first subpixel to the channel
of the transistor in the second subpixel which is positioned adjacent the
first subpixel and to which a voltage is applied, then a characteristic
shift occurs with the transistor. Here, since the second subpixel has the
light blocking member disposed between the second subpixel and the first
subpixel, the light blocking member acts to block the light emitted from
the first subpixel from entering the second subpixel. Consequently, the
characteristic shift of the transistor which is caused by incidence of
light having high energy to the channel of the transistor can be
suppressed.

[0022]With the display apparatus, since a characteristic shift of the
transistor which is caused by incidence of light having high energy to
the channel of the transistor can be suppressed, decrease of current to
flow to an electro-optical element and occurrence of a fault in picture
quality such as stripes and luminance unevenness can be suppressed.

[0023]The above and other features and advantages of the embodiment of the
present invention will become apparent from the following description and
the appended claims, taken in conjunction with the accompanying drawings
in which like parts or elements denoted by like reference symbols.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a block diagram showing a general system configuration of
an organic EL display apparatus to which an embodiment of the present
invention is applied;

[0025]FIG. 2 is a block circuit diagram showing a circuit configuration of
a pixel;

[0026]FIG. 3 is a sectional view showing an example of a sectional
structure of a pixel;

[0028]FIGS. 5A to 5D and 6A to 6D are circuit diagrams illustrating
circuit operations of the organic EL display apparatus of FIG. 1;

[0029]FIG. 7 is a characteristic diagram illustrating a subject to be
solved which arises from a dispersion of a threshold voltage of a driving
transistor;

[0030]FIG. 8 is a characteristic diagram illustrating a subject to be
solved which arises from a dispersion of a mobility of the driving
transistor;

[0031]FIGS. 9A to 9C are characteristic diagrams illustrating
relationships between a signal voltage of an image signal and
drain-source current of the driving transistor depending upon whether or
not threshold value correction and/or mobility correction are carried
out;

[0032]FIG. 10 is an equivalent circuit diagram illustrating a potential
relationship of the electrodes of a writing transistor when the white is
displayed;

[0033]FIG. 11 is a sectional view showing an example of a sectional
structure of the writing transistor;

[0034]FIG. 12 is a waveform diagram illustrating a transition waveform of
a writing scanning signal in a state wherein it is deformed at a rising
edge and a falling edge thereof;

[0035]FIG. 13 is a sectional view of a pixel section illustrating an
example of a method of preventing a pixel from being influenced by blue
light from an adjacent pixel;

[0036]FIG. 14 is a plan view showing a light blocking layout structure
according to a working example 1;

[0037]FIG. 15 is a sectional view taken along line A-A' of FIG. 14 showing
a sectional structure of the light block layout structure;

[0038]FIGS. 16 and 17 are plan views showing light blocking layout
structures according to modifications 1 and 2 to the working example 1 of
FIG. 14;

[0039]FIGS. 18 and 19 are sectional views showing light blocking layout
structures according to modifications 3 and 4 to the working example 1 of
FIG. 14;

[0040]FIG. 20 is a sectional view showing a light blocking layout
structure according to a working example 2;

[0041]FIG. 21 is a perspective view showing an appearance of a television
set to which an embodiment of the present invention is applied;

[0042]FIGS. 22A and 22B are perspective views showing an appearance of a
digital camera to which an embodiment of the present invention is applied
as viewed from the front side and the rear side, respectively;

[0043]FIG. 23 is a perspective view showing an appearance of a notebook
type personal computer to which an embodiment of the present invention is
applied;

[0044]FIG. 24 is a perspective view showing an appearance of a video
camera to which an embodiment of the present invention is applied;

[0045]FIGS. 25A and 25B are a front elevational view and a side
elevational view showing an appearance of a portable telephone set to
which an embodiment of the present invention is applied in an unfolded
state and FIGS. 25C, 25D, 25E, 25F and 25G are a front elevational view,
a left side elevational view, a right side elevational view, a top plan
view and a bottom plan view of the portable telephone set in a folded
state, respectively; and

[0046]FIG. 26 is a diagrammatic view of a transistor characteristic
illustrating a manner wherein a characteristic of a transistor varies by
a great amount when blue light is inputted to the channel of the
transistor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0047]In the following, a preferred embodiment of the present invention is
described in detail with reference to the accompanying drawings. It is to
be noted that the description is given in the following order:

[0049]FIG. 1 is a block diagram showing a general system configuration of
an active matrix display apparatus to which an embodiment of the present
invention is applied. Here, it is assumed that the active matrix display
apparatus described is an active matrix organic EL display apparatus
wherein an organic EL element which is an electro-optical element of the
current driven type whose emitted light luminance varies in response the
value of current flowing through the element is used as a light emitting
element of a pixel or pixel circuit.

[0050]Referring to FIG. 1, the organic EL display apparatus 10 shown
includes a plurality of pixels 20 each including a light emitting
element, a pixel array section 30 in which the pixels 20 are arranged
two-dimensionally in rows and columns, that is, in a matrix, and driving
sections disposed around the pixel array section 30. The driving sections
drive the pixels 20 of the pixel array section 30 to emit light.

[0051]The driving sections for the pixels 20 include a scanning driving
system including a writing scanning circuit 40 and a power supply
scanning circuit 50 and a signal supplying system including a signal
outputting circuit 60. In the organic EL display apparatus 10 of the
present embodiment, the signal outputting circuit 60 is provided on a
display panel 70 on which the pixel array section 30 is formed while the
writing scanning circuit 40 and the power supply scanning circuit 50
which form the scanning driving system are provided externally of the
display panel or substrate 70.

[0052]Here, if the organic EL display apparatus 10 is ready for
white/black display, then one pixel which makes a unit for forming a
monochromatic image corresponds to a pixel 20. On the other hand, where
the organic EL display apparatus 10 is ready for color display, one pixel
which makes a unit for forming a color image is formed from a plurality
of subpixels, each of which corresponds to a pixel 20. More particularly,
in a display apparatus for color display, one pixel is composed of three
subpixels including a subpixel for emitting red light (R), another
subpixel for emitting green light (G) and a further subpixel for emitting
blue right (B).

[0053]However, one pixel is not necessarily formed from a combination of
subpixels of the three primary colors of R, G and B but may be formed
from one or a plurality of subpixels of a color or different colors in
addition to the subpixels of the three primary colors. In particular, for
example, a subpixel for emitting white light (W) may be added to form one
pixel in order to raise the luminance, or at least one subpixel for
emitting light of a complementary color may be added to form one pixel in
order to expand the color reproduction range.

[0054]The pixels 20 are arrayed in m rows and n columns in the pixel array
section 30, and scanning lines 31-1 to 31-m and power supply lines 32-1
to 32-m are wired for the individual pixel rows along the direction of a
row, that is, along the direction along which the pixels in a pixel row
are arranged. Further, signal lines 33-1 to 33-n are wired for the
individual pixel columns along the direction of a column, that is, along
the direction along which the pixels in a pixel column are arranged.

[0055]The scanning lines 31-1 to 31-m are individually connected to output
terminals of the writing scanning circuit 40 for the corresponding rows.
The power supply lines 32-1 to 32-m are individually connected to output
terminals of the power supply scanning circuit 50 for the corresponding
rows. The signal lines 33-1 to 33-n are individually connected to output
terminals of the signal outputting circuit 60 for the corresponding
columns.

[0056]The pixel array section 30 is normally formed on a transparent
insulating substrate such as a glass substrate. Consequently, the organic
EL display apparatus 10 has a flat panel structure. A driving circuit for
each of the pixels 20 of the pixel array section 30 can be formed using
an amorphous silicon TFT (Thin Film Transistor) or a low temperature
polycrystalline silicon TFT. Where a low temperature polycrystalline
silicon TFT is used, also the writing scanning circuit 40 and power
supply scanning circuit 50 can be mounted on the display panel or
substrate 70.

[0057]The writing scanning circuit 40 is formed from a shift register
which successively shifts a start pulse sp in synchronism with a clock
pulse ck or from a like element. Upon writing of an image signal into the
pixels 20 in the pixel array section 30, the writing scanning circuit 40
successively supplies a writing scanning signal WS (WS1 to WSm) to the
scanning lines 31-1 to 31-m to successively scan (line sequential
scanning) the pixels 20 of the pixel array section 30 in a unit of a row.

[0058]The power supply scanning circuit 50 is formed from a shift register
which successively shifts the start pulse sp in synchronism with the
clock pulse ck or from a like element. The power supply scanning circuit
50 supplies a power supply potential DS (DS1 to DSm), which changes over
between a first power supply potential Vccp and a second power supply
potential Vini which is lower than the first power supply potential Vccp,
to the power supply lines 32-1 to 32-m in synchronism with line
sequential scanning by the writing scanning circuit 40. By the changeover
of the power supply potential DS between the first power supply potential
Vccp and the second power supply potential Vini, control of light
emission/no-light emission of the pixels 20 is carried out.

[0059]The signal outputting circuit 60 selects one of a signal voltage
Vsig of an image signal supplied from a signal supply line not shown and
representative of luminance information and a reference potential Vofs
and outputs the selected voltage. The reference potential Vofs
selectively outputted from the signal outputting circuit 60 is used as a
reference for the signal voltage Vsig of the image signal and
corresponds, for example, to the black level of the image signal.

[0060]The signal outputting circuit 60 may be formed using a well-known
circuit configuration, for example, of a time-division driving system.
The time-division driving system is also called selector system and
allocates a plurality of signal lines in a unit or group to one of output
terminals of a driver (not shown) which serves as a signal supplying
source. Then, the plural signal lines are successively selected
time-divisionally, and image signals outputted in a time series for the
individual output terminals of the driver are distributed and supplied
time divisionally to the selected signal lines to drive the signal lines.

[0061]In the case of a display apparatus ready for color display as an
example, image signals of R, G and B are inputted in a time series to the
signal outputting circuit 60 within one horizontal period from the driver
in a unit of three pixels of R, G and B which are positioned adjacent.
The signal outputting circuit 60 is formed from selectors or selection
switches provided corresponding to the three pixel columns of R, G and B
such that the selectors successively carry out a turning on operation
time-divisionally to write image signals of R, G and B time-divisionally
into corresponding signal lines.

[0062]While one unit here includes three pixel columns or signal lines of
R, G and B, the unit is not limited to this. In particular, since the
time-division driving method or selector method is adopted, where the
time-division number is represented by x which is an integer equal to or
greater than 2, the number of outputs of the driver and the number of
lines between the driver and the signal outputting circuit 60 and hence
between the driver and the display panel 70 can be reduced to 1/x the
number of signal lines.

[0063]The signal voltage Vsig or the reference potential Vofs selectively
outputted from the signal outputting circuit 60 is written into the
pixels 20 of the pixel array section 30 in a unit of a row through the
signal lines 33-1 to 33-n. In other words, the signal outputting circuit
60 exhibits a line sequential writing driving form wherein the signal
voltage Vsig is written in a unit of a row or line.

(Pixel Circuit)

[0064]FIG. 2 shows a particular circuit configuration of a pixel or pixel
circuit 20 used in the organic EL display apparatus 10 according to the
present embodiment.

[0065]Referring to FIG. 2, the pixel 20 includes an electro-optical
element of the current driven type whose emitted light luminance varies
in response to the value of current flowing therethrough such as, an
organic EL element 21, and a driving circuit for driving the organic EL
element 21. The organic EL element 21 is connected at the cathode
electrode thereof to a common power supply line 34 which is wired
commonly to all pixels 20.

[0066]The driving circuit for driving the organic EL element 21 includes a
driving transistor 22, a writing transistor or sampling transistor 23,
and a storage capacitor 24. Here, an N-channel TFT is used for the
driving transistor 22 and the writing transistor 23. However, this
combination of the conduction types of the driving transistor 22 and the
writing transistor 23 is a mere example, and the combination of such
conduction types is not limited to this specific combination.

[0067]It is to be noted that, where an N-channel TFT is used for the
driving transistor 22 and the writing transistor 23, an amorphous silicon
(a-Si) process can be used for the fabrication of them. Where the a-Si
process is used, reduction of the cost of a substrate on which the TFTs
are to be produced and reduction of the cost of the organic EL display
apparatus 10 can be anticipated. Further, if the driving transistor 22
and the writing transistor 23 are formed in a combination of the same
conduction type, then since the transistors 22 and 23 can be produced by
the same process, this can contribute to reduction of the cost.

[0068]The driving transistor 22 is connected at a first electrode thereof,
that is, at the source/drain electrode thereof, to the anode electrode of
the organic EL element 21 and at a second electrode thereof, that is, at
the drain/source electrode thereof, to a power supply line 32 (32-1 to
32-m).

[0069]The writing transistor 23 is connected at the gate electrode thereof
to a scanning line 31 (31-1 to 31-m). Further, the writing transistor 23
is connected at a first electrode thereof, that is, at the source/drain
electrode thereof, to a signal line 33 (33-1 to 33-n) and at a second
electrode thereof, that is, at the drain/source electrode thereof, to the
gate electrode of the driving transistor 22.

[0070]In the driving transistor 22 and the writing transistor 23, the
first electrode is a metal line electrically connected to the
source/drain region, and the second electrode is a metal line
electrically connected to the drain/source region. Further, depending
upon the relationship of the potential between the first electrode and
the second electrode, the first electrode may be the source electrode or
the drain electrode, and the second electrode may be the drain electrode
or the source electrode.

[0071]The storage capacitor 24 is connected at an electrode thereof to the
gate electrode of the driving transistor 22 and at the other electrode
thereof to the second electrode of the driving transistor 22 and the
anode electrode of the organic EL element 21.

[0072]It is to be noted that the circuit configuration of the driving
circuit for the organic EL element 21 is not limited to that which
includes the two transistors of the driving transistor 22 and the writing
transistor 23 and the one capacitor element of the storage capacitor 24.
For example, it is possible to adopt another circuit configuration
wherein an auxiliary capacitor connected at an electrode thereof to the
anode electrode of the organic EL element 21 and at the other electrode
thereof to a fixed potential is provided as occasion demands in order to
make up for shortage of the capacitance of the organic EL element 21.

[0073]In the pixel 20 having the configuration described above, the
writing transistor 23 is placed into a conducting state in response to a
High-active writing scanning signal WS applied to the gate electrode of
the writing transistor 23 through the scanning line 31 from the writing
scanning circuit 40. Consequently, the writing transistor 23 samples the
signal voltage Vsig of an image signal representative of luminance
information or the reference potential Vofs supplied from the signal
outputting circuit 60 through the signal line 33 and writes the sampled
potential into the pixel 20. The thus written signal voltage Vsig or
reference potential Vofs is applied to the gate electrode of the driving
transistor 22 and stored into the storage capacitor 24.

[0074]The driving transistor 22 operates, when the power supply potential
DS of the power supply line 32 (32-1 to 32-m) is the first power supply
potential Vccp, in a saturation region while the first electrode serves
as the drain electrode and the second electrode serves as the source
electrode. Consequently, the driving transistor 22 receives supply of
current from the power supply line 32 and drives the organic EL element
21 by current driving to emit light. More particularly, the driving
transistor 22 operates in a saturation region thereof to supply driving
current of a current value corresponding to the voltage value of the
signal voltage Vsig stored in the storage capacitor 24 to the organic EL
element 21 to drive the organic EL element 21 with the current so as to
emit light.

[0075]Further, when the power supply potential DS changes over from the
first power supply potential Vccp to the second power supply potential
Vini, the first electrode of the driving transistor 22 serves as the
source electrode while the second electrode of the driving transistor 22
serves as the drain electrode, and the driving transistor 22 operates as
a switching transistor. Consequently, the driving transistor 22 stops
supply of driving current to the organic EL element 21 by switching
operation thereof to place the organic EL element 21 into a no-light
emitting state. Thus, the driving transistor 22 has a function also as a
transistor for controlling light emission/no-light mission of the organic
EL element 21.

[0076]The switching operation of the driving transistor 22 provides a
period within which the organic EL element 21 is in a no-light emitting
state, that is, a no-light emitting period and controls the ratio between
the light emitting period and the no-light emitting period of the organic
EL element 21, that is, the duty of the organic EL element 21. By this
duty control, after-image blurring caused by emission of light from a
pixel 20 over a one-frame period can be reduced, and consequently, the
picture quality particularly of a dynamic picture can be enhanced.

[0077]The first power supply potential Vccp from between the first and
second power supply potentials Vccp and Vini selectively supplied from
the power supply scanning circuit 50 through the power supply line 32 is
a power supply potential for supplying driving current for driving the
organic EL element 21 to emit light to the organic EL element 21.
Meanwhile, the second power supply potential Vini is used to apply a
reverse bias to the organic EL element 21. This second power supply
potential Vini is set to a potential lower than the reference potential
Vofs for the signal voltage, for example, to a potential lower than
Vofs-Vth where Vth is a threshold voltage of the driving transistor 22,
preferably to a potential sufficiently lower than Vofs-Vth.

(Pixel Structure)

[0078]FIG. 3 shows a sectional structure of a pixel 20. Referring to FIG.
3, the pixel 20 is formed on a glass substrate 201 on which a driving
circuit including a driving transistor 22 and so forth is formed. The
pixel 20 is configured such that an insulating film 202, an insulating
flattening film 203 and a window insulating film 204 are formed in order
on the glass substrate 201 and an organic EL element 21 is provided at a
recessed portion 204A of the window insulating film 204. Here, from among
the components of the driving circuit, only the driving transistor 22 is
shown while the other components are omitted.

[0079]The organic EL element 21 is formed from an anode electrode 205 made
of metal or the like, an organic layer 206 formed on the anode electrode
205, and a cathode electrode 207 formed from a transparent conductive
film or the like formed commonly to all pixels on the organic layer 206.
The anode electrode 205 is formed on the bottom of the recessed portion
204A of the window insulating film 204.

[0080]In the organic EL element 21, the organic layer 206 is formed from a
hole transport layer/hole injection layer 2061, a light emitting layer
2062, an electron transport layer 2063 and an electron injection layer
(not shown) deposited in order on the anode electrode 205. If current
flows from the driving transistor 22 to the organic layer 206 through the
anode electrode 205 under the current driving by the driving transistor
22 shown in FIG. 2, then electrons and holes are recombined in the light
emitting layer 2062 in the organic layer 206, whereupon light is emitted
from the light emitting layer 2062.

[0081]The driving transistor 22 includes a gate electrode 221, a channel
formation region 225 provided at a portion of the semiconductor layer 222
opposing to the gate electrode 221 and source/drain regions 223 and 224
provided on the opposite sides of the channel formation region 225 on a
semiconductor layer 222. The source/drain region 223 is electrically
connected to the anode electrode 205 of the organic EL element 21 through
a contact hole.

[0082]Then, the organic EL element 21 is formed in a unit of a pixel on
the glass substrate 201, on which the driving circuit including the
driving transistor 22 is formed, through the insulating film 202,
insulating flattening film 203 and window insulating film 204. Then, a
sealing substrate 209 is adhered through a passivation film 208 by a
bonding agent 210, whereupon the organic EL element 21 is sealed with the
sealing substrate 209 to form the display panel 70.

[Circuit Operation of the Organic EL Display Apparatus]

[0083]Now, circuit operation of the organic EL display apparatus 10
wherein the pixels 20 having the configuration described above are
arranged two-dimensionally is described with reference to FIGS. 5A to 5D
and 6A to 6D in addition to FIG. 4.

[0084]It is to be noted that, in FIGS. 5A to 5D and 6A to 6D, the writing
transistor 23 is represented by a symbol of a switch for simplified
illustration. Further, as well known in the art, the organic EL element
21 has equivalent capacitance or parasitic capacitance Cel. Accordingly,
also the equivalent capacitance Cel is shown in FIGS. 5A to 5D and 6A to
6D.

[0085]In FIG. 4, a variation of the potential of the writing scanning
signal WS of a scanning line 31 (31-1 to 31-m), a variation of the
potential of the power supply potential DS of a power supply line 32
(32-1 to 32-m) and variations of the gate potential Vg and the source
potential Vs of the driving transistor 22.

<<Light Emitting Period within the Preceding Frame>>

[0086]In FIG. 4, prior to time t1, a light emitting period of the organic
EL element 21 within the preceding frame or field is provided. Within the
light emitting period of the preceding frame, the power supply potential
DS of the power supply line 32 has a first power supply potential
(hereinafter referred to as "high potential") Vccp and the writing
transistor 23 is in a non-conductive state.

[0087]The driving transistor 22 is designed such that, at this time, it
operates in a saturation region. Consequently, driving current or
drain-source current Ids corresponding to the gate-source voltage Vgs of
the driving transistor 22 is supplied from the power supply line 32 to
the organic. EL element 21 through the driving transistor 22.
Consequently, the organic EL element 21 emits light with a luminance
corresponding to the current value of the driving current Ids.

<<Threshold Value Correction Preparation Period>>

[0088]At time t1, a new frame of line sequential scanning, that is, a
current frame, is entered. Then, the potential DS of the power supply
line 32 changes over from the high potential Vccp to a second power
supply voltage (hereinafter referred to as "low potential") Vini, which
is sufficiently lower than Vofs-Vth, with respect to the reference
potential Vofs of the signal line 33 as seen from FIG. 5B.

[0089]Here, the threshold voltage of the organic EL element 21 is
represented by Vthe1, and the potential of the common power supply line
34, that is, the cathode potential, is represented by Vcath. At this
time, if the low potential Vini satisfies Vini<Vthe1+Vcath, then since
the source potential Vs of the driving transistor 22 becomes
substantially equal to the low potential Vini, the organic EL element 21
is placed into a reversely biased state and stops the emission of light.

[0090]Then, when the potential WS of the scanning line 31 changes from the
low potential side to the high potential side at time t2, the writing
transistor 23 is placed into a conducting state as seen from FIG. 5C. At
this time, since the reference potential Vofs is supplied from the signal
outputting circuit 60 to the signal line 33, the gate potential Vg of the
driving transistor 22 becomes equal to the reference potential Vofs.
Meanwhile, the source potential Vs of the driving transistor 22 is equal
to the low potential Vini sufficiently lower than the reference potential
Vofs.

[0091]At this time, the gate-source voltage Vgs of the driving transistor
22 is Vofs-Vini. Here, if Vofs-Vini is not sufficiently greater than the
threshold potential Vth of the driving transistor 22, then a threshold
value correction process hereinafter described cannot be carried out, and
therefore, it is necessary to establish the potential relationship of
Vofs-Vini>Vth.

[0092]In this manner, the process of fixing or finalizing the gate
potential Vg of the driving transistor 22 to the reference potential Vofs
and the source potential Vs of the driving transistor 22 to the low
potential Vini to initialize them is a process of preparation (threshold
value correction preparation) before a threshold value correction process
hereinafter described is carried out. Accordingly, the reference
potential Vofs and the low potential Vini become initialization
potentials for the gate potential Vg and the source potential Vs of the
driving transistor 22, respectively.

<<Threshold Value Correction Period>>

[0093]Then, if the potential DS of the power supply line 32 changes over
from the low potential Vini to the high potential Vccp at time t3 as seen
in FIG. 5D, then a threshold value correction process is started in a
state wherein the gate potential Vg of the driving transistor 22 is
maintained. In particular, the source potential Vs of the driving
transistor 22 begins to rise toward the potential of the difference of
the threshold potential Vth of the driving transistor 22 from the gate
potential Vg.

[0094]Here, the process of varying the source potential Vs toward the
potential of the difference of the threshold potential Vth of the driving
transistor 22 from the reference potential Vofs with reference to the
initialization potential Vofs at the gate electrode of the driving
transistor 22 is hereinafter referred to as threshold value correction
process. As the threshold value correction process progresses, the
gate-source voltage Vgs of the driving transistor 22 soon converges to
the threshold potential Vth of the driving transistor 22. The voltage
corresponding to the threshold potential Vth is stored into the storage
capacitor 24.

[0095]It is to be noted that it is necessary to allow, within a period
within which the threshold value correction process is carried out, that
is, within a threshold value correction period, current to wholly flow to
the storage capacitor 24 side but not to flow to the organic EL element
21 side. To this end, the potential Vcath of the common power supply line
34 is set so that the organic EL element 21 has a cutoff state.

[0096]Then, the potential WS of the scanning line 31 changes to the low
potential side at time t4, whereupon the writing transistor 23 is placed
into a non-conducting state as seen in FIG. 6A. At this time, the gate
electrode of the driving transistor 22 is electrically disconnected from
the signal line 33 and enters a floating state. However, since the
gate-source voltage Vgs is equal to the threshold potential Vth of the
driving transistor 22, the driving transistor 22 remains in a cutoff
state. Accordingly, the amount of the drain-source current Ids flowing to
the driving transistor 22 is very small.

<<Signal Writing & Mobility Correction Period>>

[0097]Then at time t5, the potential of the signal line 33 changes over
from the reference potential Vofs to the signal voltage Vsig of the image
signal as seen in FIG. 6B. Then at time t6, the potential WS of the
scanning line 31 changes to the high potential side, whereupon the
writing transistor 23 is placed into a conducting state as seen in FIG.
6C to sample and write the signal voltage Vsig of the image signal into
the pixel 20.

[0098]By the writing of the signal voltage Vsig by the writing transistor
23, the gate potential Vg of the driving transistor 22 becomes equal to
the signal voltage Vsig. Then, upon driving of the driving transistor 22
with the signal voltage Vsig of the image signal, the threshold potential
Vth of the driving transistor 22 is canceled with the voltage
corresponding to the threshold potential Vth stored in the storage
capacitor 24. Details of the principle of the threshold value
cancellation are hereinafter described in detail.

[0099]At this time, the organic EL element 21 remains in a cutoff state,
that is, in a high-impedance state. Accordingly, current flowing from the
power supply line 32 to the driving transistor 22 in response to the
signal voltage Vsig of the image signal, that is, the drain-source
current Ids, flows into the equivalent capacitance Cel. Charging of the
equivalent capacitance Cel of the organic EL element 21 is started.

[0100]By the charging of the equivalent capacitance Cel, the source
potential Vs of the driving transistor 22 rises together with lapse of
time. At this time, a dispersion of the threshold potential Vth of the
driving transistor 22 for each pixel is canceled already, and the
drain-source current Ids of the driving transistor 22 exhibits a value
which relies upon the mobility μ of the driving transistor 22.

[0101]Here, it is assumed that the ratio of the storage voltage Vgs of the
storage capacitor 24 to the signal voltage Vsig of the image signal is 1,
which is an ideal value. The ratio of the storage voltage Vgs to the
signal voltage Vsig is hereinafter referred to sometimes as write gain.
In this instance, when the source potential Vs of the driving transistor
22 rises to the potential of Vofs-Vth+ΔV, the gate-source voltage
Vgs of the driving transistor 22 becomes Vsig-Vofs+Vth-ΔV.

[0102]In particular, the rise amount ΔV of the source potential Vs
of the driving transistor 22 acts so as to be subtracted from the voltage
stored in the storage capacitor 24, that is, from Vsig-Vofs+Vth. Or in
other words, the rise amount ΔV of the source potential Vs acts so
as to discharge the accumulated charge of the storage capacitor 24, and
therefore, is negatively fed back. Accordingly, the rise amount ΔV
of the source potential Vs of the driving transistor 22 is a feedback
amount in the negative feedback.

[0103]By applying negative feedback of the feedback amount ΔV in
accordance with the driving current Ids flowing through the driving
transistor 22 to the gate-source voltage Vgs in this manner, the
dependency of the drains-source current Ids of the driving transistor 22
upon the mobility μ can be canceled. This process of canceling the
dependency upon the mobility μ is a mobility correction process of
correcting the dispersion of the mobility μ of the driving transistor
22 for each pixel.

[0104]More particularly, since the drain-source current Ids increases as
the signal amplitude Vin (=Vsig-Vofs) of the image signal to be written
into the gate electrode of the driving transistor 22 increases, also the
absolute value of the feedback amount ΔV of the negative feedback
increases. Accordingly, a mobility correction process in accordance with
the emitted light luminance level is carried out.

[0105]Further, if it is assumed that the signal amplitude Vin of the image
signal is fixed, then since also the absolute value of the feedback
amount ΔV of the negative feedback increases as the mobility μ
of the driving transistor 22 increases, a dispersion of the mobility μ
for each pixel can be removed. Accordingly, the feedback amount ΔV
of the negative feedback can be regarded also as a correction amount of
mobility correction. Details of the principle of the mobility correction
are hereafter described.

<<Light Emitting Period>>

[0106]Then, the potential WS of the scanning line 31 changes to the low
potential side at time t7, whereupon the writing transistor 23 is placed
into a non-conducting state as seen from FIG. 6D. Consequently, the gate
potential of the driving transistor 22 is placed into a floating state
because it is electrically disconnected from the signal line 33.

[0107]Here, when the gate electrode of the driving transistor 22 is in a
floating state, since the storage capacitor 24 is connected between the
gate and the source of the driving transistor 22, also the gate potential
Vg varies in an interlocked relationship or following up relationship
with a variation of the source potential Vs of the driving transistor 22.
An operation of the gate potential Vg of the driving transistor 22 which
varies in an interlocked relationship with a variation of the source
potential Vs in this manner is hereinafter referred to as bootstrap
operation by the storage capacitor 24.

[0108]When the gate electrode of the driving transistor 22 is placed into
a floating state and the drain-source current Ids of the driving
transistor 22 simultaneously begins to flow to the organic EL element 21,
the anode potential of the organic EL element 21 rises in response to the
drain-source current Ids.

[0109]Then, when the anode potential of the organic EL element 21 exceeds
Vthe1+Vcath, driving current begins to flow to the organic EL element 21,
and consequently, the organic EL element 21 starts emission of light.
Further, the rise of the anode potential of the organic EL element 21 is
nothing but a rise of the source potential Vs of the driving transistor
22. As the source potential Vs of the driving transistor 22 rises, also
the gate potential Vg of the driving transistor 22 rises in an
interlinked relationship by the bootstrap operation of the storage
capacitor 24.

[0110]At this time, if it is assumed that the bootstrap gain is 1 in an
ideal state, then the rise amount of the gate potential Vg is equal to
the rise amount of the source potential Vs. Therefore, during the light
emitting period, the gate-source voltage Vgs of the driving transistor 22
is kept fixed at Vsig-Vofs+Vth-ΔV. Then, at time t8, the potential
of the signal line 33 changes over from the signal voltage Vsig of the
image signal to the reference potential Vofs.

[0111]In a series of circuit operations described above, the processing
operations of threshold value correction preparation, threshold value
correction, writing of the signal voltage Vsig (signal writing) and
mobility correction are executed within one horizontal scanning period
(1H). Meanwhile, the processing operations of signal writing and mobility
correction are executed in parallel within the period from time t6 to
time t7.

[0112]It is to be noted here that, although the example described above
adopts the driving method wherein the threshold value correction process
is executed only once, this driving method is a mere example and the
driving method to be adopted is not limited to this. For example, it is
possible to adopt a driving method wherein the threshold value correction
process is executed within a 1H period within which it is carried out
together with the mobility correction and signal writing processes and is
further executed by a plural number of times divisionally in a plurality
of horizontal scanning periods preceding to the 1H period.

[0113]Where the driving method of the divisional threshold value
correction just described is adopted, even if the time allocated to one
horizontal scanning period is decreased by increase of the number of
pixels by increase of the definition, a sufficient period of time can be
assured as the threshold value correction period over a plurality of
horizontal scanning periods. Consequently, the threshold value correction
process can be carried out with certainty.

(Principle of the Threshold Value Cancellation)

[0114]Here, the principle of threshold value cancellation, that is, of the
threshold value correction, by the driving transistor 22 is described.
The threshold value correction process is a process of varying the source
voltage Vs of the driving transistor 22 toward a potential of the
difference of the threshold voltage Vth of the driving transistor 22 from
the initialization potential Vofs for the gate potential Vg of the
driving transistor 22 with reference to the initialization potential Vofs
as described hereinabove.

[0115]The driving transistor 22 operates as a constant current source
because it is designed so as to operate in a saturation region. Since the
driving transistor 22 operates as a constant current source, the organic
EL element 21 is supplied with fixed drain-source current or driving
current Ids given by the following expression (1):

Ids=(1/2)μ(W/L)Cox(Vgs-Vth)2 (1)

where W is the channel width of the driving transistor 22, L the channel
length, and Cox the gate capacitance per unit area.

[0116]FIG. 7 illustrates a characteristic of the drain-source current Ids
with respect to the gate-source voltage Vgs of the driving transistor 22.

[0117]As seen from the characteristic diagram of FIG. 7, if a cancellation
process for a dispersion of the threshold potential Vth of the driving
transistor 22 for each pixel is not carried out, then when the threshold
potential Vth is Vth1, the drain-source current Ids corresponding to the
gate-source potential Vgs becomes Ids1.

[0118]In contrast, when the threshold potential Vth is Vth2
(Vth2>Vth1), the drain-source current Ids corresponding to the same
gate-source voltage Vgs becomes Ids2 (Ids2<Ids1). In other words, if
the threshold potential Vth of the driving transistor 22 fluctuates, then
even if the gate-source voltage Vgs is fixed, the drain-source current
Ids fluctuates.

[0119]On the other hand, in the pixel or pixel circuit 20 having the
configuration described above, the gate-source voltage Vgs of the driving
transistor 22 upon light emission is Vsig-Vofs+Vth-ΔV. Accordingly,
by substituting this into the expression (1), the drain-source current
Ids is represented by the following expression (2):

Ids=(1/2)μ(W/L)Cox(Vsig-Vofs-ΔV)2 (2)

[0120]In particular, the term of the threshold potential Vth of the
driving transistor 22 is canceled, and the drain-source current Ids to be
supplied from the driving transistor 22 to the organic EL element 21 does
not rely upon the threshold potential Vth of the driving transistor 22.
As a result, even if the threshold potential Vth of the driving
transistor 22 varies for each pixel due to a dispersion of the
fabrication process or aged deterioration of the driving transistor 22,
the drain-source current Ids does not vary, and consequently, the emitted
light luminance of the organic EL element 21 can be kept fixed.

(Principle of the Mobility Correction)

[0121]Now, the principle of the mobility correction of the driving
transistor 22 is described. The mobility correction process is a process
of applying negative feedback of the correction amount ΔV
corresponding to the drain-source current Ids flowing to the driving
transistor 22 to the potential difference between the gate and the source
of the driving transistor 22. By the mobility correction process, the
dependency of the drain-source current Ids of the driving transistor 22
upon the mobility μ can be canceled.

[0122]FIG. 8 illustrates characteristic curves of a pixel A whose driving
transistor 22 has a relatively high mobility μ and a pixel B whose
driving transistor 22 has a relatively low mobility μ for comparison.
Where the driving transistor 22 is formed from a polycrystalline silicon
thin film transistor or the like, it cannot be avoided that the mobility
μ disperses among pixels like the pixel A and the pixel B.

[0123]It is assumed here that, in a state wherein the pixel A and the
pixel B have a dispersion in mobility μ therebetween, the signal
amplitudes Vin (=Vsig-Vofs) of an equal level are written into the gate
electrodes of the driving transistors 22 in the pixels A and B. In this
instance, if correction of the mobility μ is not carried out at all,
then a great difference appears between the drain-source current Ids1'
flowing through the pixel A having the high mobility μ and the
drain-source current Ids2' flowing through the pixel B having the low
mobility μ. If a great difference in the drain-source current Ids
appears between different pixels originating from the dispersion of the
mobility μ among the pixels in this manner, then uniformity of the
screen image is damaged.

[0124]Here, as apparent from the transistor characteristic expression of
the expression (1) given hereinabove, where the mobility μ is high,
the drain-source current Ids is great. Accordingly, the feedback amount
ΔV in the negative feedback increases as the mobility μ
increases. As seen from FIG. 8, the feedback amount ΔV1 in the
pixel A of the high mobility μ is greater than the feedback amount
ΔV2 in the pixel B having the low mobility μ.

[0125]Therefore, if negative feedback is applied to the gate-source
voltage Vgs with the feedback amount ΔV in accordance with the
drain-source current Ids of the driving transistor 22 by the mobility
correction process, then the negative feedback amount increases as the
mobility μ increases. As a result, the dispersion of the mobility μ
among the pixels can be suppressed.

[0126]In particular, if correction of the feedback amount ΔV1 is
applied in the pixel A having the high mobility μ, then the
drain-source current Ids drops by a great amount from Ids1' to Ids1. On
the other hand, since the feedback amount ΔV2 in the pixel B having
the low mobility μ is small, the drain-source current Ids decreases
from Ids2' to Ids2 and does not drop by a great amount. As a result, the
drain-source current Ids1 in the pixel A and the drain-source current
Ids2 in the pixel B become substantially equal to each other, and
consequently, the dispersion of the mobility μ among the pixels is
corrected.

[0127]In summary, where the pixel A and the pixel B which are different in
the mobility μ therebetween are considered, the feedback amount
ΔV1 in the pixel A having the high mobility μ is greater than
the feedback amount ΔV2 in the pixel B having the low mobility
μ. In short, as the mobility μ increases, the feedback amount
ΔV increases and the reduction amount of the drain-source current
Ids increases.

[0128]Accordingly, if the negative feedback is applied to the gate-source
voltage Vgs with the feedback amount ΔV in accordance with the
drain-source current Ids of the driving transistor 22, then the current
value of the drain-source current Ids is uniformized among the pixels
which are different in the mobility μ from each other. As a result,
the dispersion of the mobility μ among the pixels can be corrected.
Thus, the process of applying negative feedback to the gate-source
voltage Vgs of the driving transistor 22 with the feedback amount
ΔV in accordance with the current flowing through the driving
transistor 22, that is, with the drain-source current Ids, is the
mobility correction process.

[0129]Here, a relationship between the signal voltage or sampling
potential Vsig of the image signal and the drain-source current Ids of
the driving transistor 22 depending upon whether or not threshold value
correction and mobility correction are carried out in the pixel or pixel
circuit 20 shown in FIG. 2 is described with reference to FIGS. 9A to 9C.

[0130]FIG. 9A illustrates the relationship in a case wherein none of the
threshold value correction process and the mobility correction process is
carried out, and FIG. 9B illustrates the relationship in another case
wherein only the threshold value correction process is carried out
without carrying out the mobility correction process while FIG. 9C
illustrates the relationship in a further case wherein both of the
threshold value correction process and the mobility correction process
are carried out. As seen in FIG. 9A, when none of the threshold value
correction process and the mobility correction process is carried out,
the drain-source current Ids is much different between the pixels A and B
arising from a dispersion of the threshold potential Vth and the mobility
μ between the pixels A and B.

[0131]In contrast, where only the threshold value correction process is
carried out, although the dispersion of the drain-source current Ids can
be reduced to some degree as seen in FIG. 9B, the difference in the
drain-source current Ids between the pixels A and B arising from the
dispersion of the mobility μ between the pixels A and B remains. Then,
if both of the threshold value correction process and the mobility
correction process are carried out, then the difference in the
drain-source current Ids between the pixels A and B arising from the
dispersion of the mobility μ for each of the pixels A and B can be
almost eliminated as seen in FIG. 9C. Accordingly, at any gradation, a
luminance dispersion among the organic EL elements 21 does not appear,
and a display image of favorable picture quality can be obtained.

[0132]Further, since the pixel 20 shown in FIG. 2 has a function of a
bootstrap operation by the storage capacitor 24 described hereinabove in
addition to the correction functions for threshold value correction and
mobility correction, the following operation and effects can be achieved.

[0133]In particular, even if the source potential Vs of the driving
transistor 22 varies together with aged deterioration of the I-V
characteristic of the organic EL element 21, the gate-source voltage Vgs
of the driving transistor 22 can be kept fixed by a bootstrap operation
by the storage capacitor 24. Accordingly, the current flowing through the
organic EL element 21 does not vary but is fixed. As a result, since also
the emitted light luminance of the organic EL element 21 is kept fixed,
even if the I-V characteristic of the organic EL element 21 undergoes
aged deterioration, image display which is free from luminance
deterioration by the aged deterioration can be implemented.

(Fault by a Shift of the Threshold Voltage of the Writing Transistor)

[0134]Here, the operating point of the writing transistor 23 when the
organic EL element 21 emits light, particularly when the organic EL
element 21 displays the white, is studied. As apparent from the circuit
operation described above, after writing of the signal voltage Vsig of
the image signal ends and the writing transistor 23 enters a
non-conducting state, the gate voltage Vg of the driving transistor 22
rises in an interlocking relationship with a rise of the source voltage
Vs through a bootstrap operation. Therefore, the gate voltage Vg of the
driving transistor 22 becomes higher than the signal voltage Vsig.

[0135]On the other hand, if a configuration for applying the reference
potential Vofs for the initialization of the gate voltage Vg of the
driving transistor 22 is applied through the signal line 33 in order to
execute the threshold value correction process is adopted, then the
potential of the signal line 33 exhibits repetitions of changeover
between the reference voltage Vofs and the signal voltage Vsig in a
period of 1H.

[0136]FIG. 10 illustrates a potential relationship of the electrodes of
the writing transistor 23 upon white display. Upon white display, an off
voltage Vssws for placing the writing transistor 23 into a non-conducting
state is applied to the gate electrode G of the writing transistor 23 and
the reference voltage Vofs is applied to the source electrode S of the
writing transistor 23 while a white voltage Vw corresponding to the white
gradation is applied to the drain electrode D of the writing transistor
23. The off voltage Vssws, reference voltage Vofs and white voltage Vw
have a voltage relationship of Vssws<Vofs<Vw.

[0137]FIG. 11 shows an example of a sectional structure of the writing
transistor 23. Referring to FIG. 11, a gate electrode 231 is formed from
molybdenum (Mo) or the like on a substrate which corresponds to the glass
substrate 201 shown in FIG. 3, and a semiconductor layer 233 of, for
example, amorphous silicon (a-Si) is layered on the gate electrode 231
with a gate insulating film 232 interposed therebetween.

[0138]A portion of the semiconductor layer (a-Si) 233 which opposes to the
gate electrode 231 forms a channel formation region. An insulating film
234 is formed on the channel formation region. A source electrode 235 and
a drain electrode 236 both made of aluminum (Al) or the like are
electrically connected to a source region and a drain region of the
semiconductor layer 233, respectively, between which the channel
formation region is sandwiched.

[0139]In the writing transistor 23 having the configuration described
above, when the white is to be displayed, the off voltage Vssws is
applied to the gate electrode 231 and the white voltage Vw is applied to
the drain electrode 236 so that a high electric field is formed between
the gate electrode 231 and the drain electrode 236. Here, the drain
voltage of the writing transistor 23 is equal to the gate voltage of the
driving transistor.

[0140]If an electric field continues to be generated between the gate
electrode 231 and the drain electrode 236 of the writing transistor 23,
then electrons in the semiconductor layer 233 which are to form the
channel are trapped in the insulating film 234 positioned above the
semiconductor layer 233 and tend to generate a reverse electric field in
a direction in which the electric field is to be canceled. Since the
trapped electrons exist also when the writing transistor 23 conducts, the
threshold voltage Vthws of the writing transistor 23 is shifted or
fluctuated to the negative side by the reverse electric field. The
phenomenon that the threshold voltage Vthws is shifted to the negative
side appears conspicuously as time passes.

[0141]Incidentally, as the increase in size and definition of a display
panel advances, the wiring line resistance and the parasitic capacitance
of the scanning line 31 for transmitting the writing scanning signal WS
in the form of a pulse to be applied to the gate electrode of the writing
transistor 23 increase. Then, where the wiring line resistance or the
parasitic capacitance of the scanning line 31 increases, as the distance
from the input end of the display panel 70 increases, the waveform of the
writing scanning signal WS becomes blunt.

[0142]Meanwhile, the mobility correction process is executed in parallel
to the writing process of the signal voltage Vsig of the image signal by
the writing transistor 23. As apparent from the timing waveform diagram
of FIG. 4, the mobility correction period, that is, the signal writing
period, depends upon the waveform of the writing scanning signal WS.
Therefore, if the threshold voltage Vthws of the writing transistor 23
shifts to the negative side in a state wherein the waveform of the
writing scanning signal WS is blunt, then the mobility correction time
when the white is to be displayed or the black is to be displayed becomes
longer by a period of time corresponding to the shift amount of the
threshold voltage.

[0143]FIG. 12 illustrates a transition waveform of the writing scanning
signal WS in a state wherein it is blunt at a rising edge and a fall edge
thereof. Referring to FIG. 12, reference character VsigW denotes a white
signal voltage corresponding to the white gradation; VsigB a black signal
voltage corresponding to the black gradation; and ΔVthws a shift
amount of the threshold voltage Vthws of the writing transistor 23.

[0144]As can be seen apparently from the waveform diagram of FIG. 12,
particularly when the white or the black is displayed, if the threshold
voltage Vthws of the writing transistor 23 shifts by the shift amount
ΔVthws to the negative side, then the mobility correction period
increases by the shift amount ΔVthws of the threshold voltage
Vthws. This variation of the threshold voltage Vthws appears
conspicuously particularly at a falling edge of the waveform of the
writing scanning signal WS. The reason is such as described below.

[0145]As seen from the waveform diagram of FIG. 12, in the transition
waveform of the writing scanning signal WS, a transition ending portion
of the rising/falling edges exhibits a higher degree of blunting of the
waveform than a transition starting portion of the rising/falling edges.
The amplitude of the white signal voltage VsigW is equal to or smaller
than one half that of the writing scanning signal WS. Accordingly, as
apparently seen from the waveform diagram of FIG. 12, the variation of
the mobility correction period arising from the shift of the threshold
voltage Vthws of the writing transistor 23 to the negative side appears
conspicuously particularly at the falling edge of the waveform of the
writing scanning signal WS.

[0146]Further, as apparent from the foregoing description of the circuit
operation, the mobility correction process is carried out while the
source voltage Vs of the driving transistor 22 is raised. Therefore, as
the mobility correction period increases, the rises of the source voltage
Vs of the driving transistor 22 increases. Consequently, the gate-source
voltage Vgs of the driving transistor 22 drops and the current to flow to
the organic EL element 21 decreases, and as a result, the emitted light
luminance decreases as time passes or such a picture quality defect as
stripes or luminance unevenness appears.

[0147]Further, as described hereinabove, also when light of high energy is
inputted to the channel of the writing transistor 23, the threshold
voltage of the writing transistor 23 shifts to the negative side (refer
to FIG. 26). Where the R, G and B pixels (subpixels) are disposed such
that the B pixel is positioned centrally, the B pixel is influenced by
blue light only when it itself emits light.

[0148]On the other hand, since the R and G pixels are positioned adjacent
the B pixel, they are influenced by emitted light of the B pixel even
when they themselves emit no light. At this time, not only the writing
transistor 23 but also the driving transistor 22 is influenced by blue
light such that the characteristic thereof is shifted. Where not only an
influence of self light emission but also an influence of light emission
of adjacent pixels is had, it is very difficult to compensate for the
current variation in correction processes such as the mobility correction
process.

[0149]As a method of preventing an influence of blue light from an
adjacent pixel, a method seems applicable wherein the writing transistor
23 is covered with a metal wiring layer 301 in the same layer as that of
the anode electrode of the organic EL element 21 as seen in FIG. 13 to
block blue light. However, with the method described, although a light
blocking effect can be expected to some degree, since it is necessary to
form a flattening film 302, which corresponds to the insulating
flattening film 203 shown in FIG. 3, with an increased thickness in order
to maintain the flatness, full light blocking cannot be expected even if
the metal wiring layer 301 is disposed on the writing transistor 23.

Characteristics of the Present Embodiment

[0150]Therefore, the present embodiment adopts the following pixel light
blocking layout in an organic EL display apparatus wherein a plurality of
subpixels which form one pixel which forms a unit in formation of a color
image are disposed adjacent each other. In particular, the present
embodiment adopts a light blocking layout structure wherein, at least for
a second subpixel from among the plural subpixels which is positioned
adjacent a first subpixel which emits light of the shortest wavelength, a
light blocking member having a width greater than the channel length or
the channel width of a transistor which forms the second subpixel is
provided so as to be positioned between the first and second subpixels.

[0151]Although description here is given of a case wherein the plural
subpixels which form one pixel which forms a unit in formation of a color
image are formed, for example, from a combination of R, G and B pixels or
subpixels, they are not limited to the specific combination. In the case
of the combination of R, G and B pixels, the emitted light from the B
pixel has the shortest wavelength. Accordingly, the B pixel serves as the
first subpixel, and each of the R and G pixels serves as the second
subpixel.

[0152]The transistor which forms the second subpixel may be, for example,
the writing transistor 23. However, as described above, not only a
characteristic shift of the writing transistor 23 is caused by an
influence of blue light of the B pixel, but also a characteristic shift
of the driving transistor 22 is caused by an influence of blue light.
Accordingly, the transistor which serves as the second subpixel is not
limited to the writing transistor 23.

[0153]Where a light blocking member is provided for the second subpixel
with respect to the first subpixel such that the width thereof is greater
than the channel length or the channel width of the transistor which
forms the second subpixel as described above, light emitted from the
first subpixel can be blocked with certainty. Accordingly, a
characteristic shift caused by inputting of light having high energy to
the channel of the transistor which forms the second subpixel,
particularly a shift of the threshold voltage Vth to the negative side,
can be suppressed.

[0154]Since a shift of the threshold voltage of the writing transistor 23
is suppressed, the fluctuation of the mobility correction period or
signal writing period which depends upon the waveform of the writing
scanning signal WS can be reduced. Where the fluctuation of the mobility
correction period is reduced, the rise of the source voltage Vs of the
driving transistor 22 arising from the fluctuation can be suppressed.
Therefore, reduction of the current to flow to the organic EL element 21
is suppressed, and consequently, reduction of the emitted light luminance
with respect to time and occurrence of a fault in picture quality such as
stripes or luminance unevenness can be suppressed.

[0155]Such effects as described above can be obtained from the light
blocking layout structure for a pixel according to the present
embodiment. In the following, particular working examples of the light
blocking layout structure are described.

1-1. Working Example 1

[0156]FIG. 14 is a plan view showing the light blocking layout structure
according to a working example 1. Here, as an example, the light blocking
layout structure exhibits a color array wherein R, G and B pixels or
subpixels 20R, 20G and 20B are arranged such that the B pixel 20B is
positioned centrally and the R and G pixels 20R and 20B are positioned on
the opposite sides of the B pixel 20B. FIG. 15 shows a sectional
structure of the light blocking layout structure taken along line A-A' of
FIG. 14.

[0157]Since the B pixel 20B is disposed centrally as seen in FIG. 14,
transistors in the R and G pixels 20R and 20G positioned on the opposite
sides of the B pixel 20B are influenced by irradiation of blue light
emitted from the B pixel 20B. In order to prevent incidence of blue light
from the B pixel 20B, a metal wiring layer 301G is provided in the layer
same as that of the anode electrode of the G pixel 20G on the writing
transistor 23, on the organic EL element 21 side of FIG. 3, with the
flattening film 302 interposed therebetween similarly as in the case of
the light blocking layout structure described hereinabove with reference
to FIG. 13. The metal wiring layer 301G is formed from a metal material
having high reflectivity such as aluminum and same as that of the wiring
line for the anode electrode.

[0158]The G pixel 20G further includes a light blocking member 303G
provided in parallel to the longitudinal direction of the G pixel 20G
between the G pixel 20G and the B pixel 20B in a region of the metal
wiring layer 301G as shown in FIGS. 14 and 15. The light blocking member
303G has a width greater than the channel width of the writing transistor
23 as apparently seen particularly from FIG. 14 and is embedded in a hole
formed in the metal wiring layer 301G. As the material of the light
blocking member 303G, for example, a material same as that of the metal
wiring layer 301G, that is, a metal material having high reflectivity
such as aluminum, is used.

[0159]While the light blocking layout structure of the G pixel 20G is
described here, also the R pixel 20R has a light blocking layout
structure basically same as that of the G pixel 20G. The light blocking
layout structure of the G pixel 20G and the light blocking layout
structure of the R pixel 20R are symmetrical to each other with respect
to the center line of the B pixel 20B.

[0160]Where the light blocking members 303G and 303R are provided between
the B pixel 20B and the R and G pixels 20R and 20B positioned adjacent to
and on the opposite sides of the B pixel 20B in this manner,
respectively, blue light emitted from the B pixel 20B can be blocked with
certainty so as not to be inputted to the pixels 20R and 20G. Therefore,
the characteristic shift by an influence of irradiation of blue light
upon the channel of the writing transistor 23 can be suppressed low, and
consequently, reduction of the current to flow to the organic EL element
21 and occurrence of a fault in picture quality such as stripes or
luminance unevenness can be suppressed.

Modification 1 to the Working Example 1

[0161]FIG. 16 is a plan view showing a light blocking layout structure
according to a modification 1 to the working example 1. In the light
blocking layout structure according to the modification 1, light blocking
members 303B-1 and 303B-2 having a width greater than the channel width
of the writing transistor 23 are provided between R and G pixels 20R and
20B positioned adjacent to and on the opposite sides of the B pixel 20B.

[0162]With the light blocking layout structure according to the
modification 1, blue light emitted from the B pixel 20B can be prevented
from being reflected by the light blocking members 303G and 303R of the
adjacent pixels and entering the B pixel 20B. Accordingly, not only with
regard to the R and G pixels 20R and 20B but also with regard to the B
pixel 20B, the characteristic shift by an influence of irradiation of
blue light can be suppressed to a small amount.

Modification 2 to the Working Example 1

[0163]FIG. 17 is a plan view showing a light blocking layout structure
according to a modification 2 to the working example 1. Referring to FIG.
17, in the light blocking layout structure according to present
modification 2, each of the pixels 20G, 20B and 20R is blocked against
light by light blocking members 303-1 to 303-4 in the four directions of
leftward, rightward, upward and downward directions of the writing
transistor 23.

[0164]In particular, the writing transistor 23 in the G pixel 20G is
blocked on the left and right thereof against light by the light blocking
members 303G-1 and 303G-2. The light blocking members 303G-1 and 303G-2
have a width greater than the channel width of the writing transistor 23.
Further, the writing transistor 23 is blocked on the upper and lower
sides thereof against light by the light blocking members 303G-3 and
303G-4. The light blocking members 303G-3 and 303G-4 have a width greater
than the channel length of the writing transistor 23.

[0165]Also for the B and R pixels 20B and 20R, a light blocking layout
structure basically similar to that for the G pixel 20G is provided.

[0166]With the light blocking layout structure according to the present
modification 2, the writing transistors 23 in the pixels 20G, 20B and 20R
can be shielded optically substantially fully from blue light emitted
from the B pixel 20B. Accordingly, with regard to each of the pixels 20G,
20B and 20R, the characteristic shift by an influence of irradiation of
blue light can be suppressed with a higher degree of certainty.

Modification 3 to the Working Example 1

[0167]FIG. 18 is a sectional view showing a light blocking layout
structure according to a modification 3 to the working example 1.
Referring to FIG. 18, in the light blocking layout structure according to
the third modification, a lower end portion of a light blocking member
303B is embedded in a gate insulating film 232 which corresponds to the
insulating film 202 shown in FIG. 3.

[0168]While the light blocking member 303B for the B pixel 20B is shown in
FIG. 18, also the light blocking members 303B-1 and 303B-2 in the
modification 1 and the insulating films 303G-1 to 303G-4 in the
modification 2 can be embedded at a lower end portion thereof in a gate
insulating film 304. The foregoing description applies similarly to the B
and R pixels 20B ad 20R.

[0169]With the light blocking layout structure according to the present
modification 3, since the lower end portion of the light blocking member
303B is embedded in the gate insulating film 304, also leakage light from
between the lower end portion of the light blocking member 303B and the
gate insulating film 304 can be blocked with certainty. Accordingly, in
each of the pixels 20G, 20B and 20R, the characteristic shift by an
influence of irradiation of blue light can be suppressed with certainty.

Modification 4 to the Working Example 1

[0170]FIG. 19 is a sectional view showing a light blocking layout
structure according to a modification 4 to the working example 1.
Referring to FIG. 19, in the light blocking layout structure according to
the modification 4 to the working example 1, a lower end portion of the
light blocking member 303B is electrically connected to a wiring line 305
for the source electrode of the driving transistor 22.

[0171]With the light blocking layout structure according to the present
modification 4, since the light blocking member 303B establishes electric
connection between the source electrode of the driving transistor 22 and
the anode electrode of the organic EL element 21, the light blocking
member 303B can be used also as a contact portion for the electric
connection. Consequently, the light blocking layout structure against
blue light can be implemented without inviting complication of the pixel
structure by the provision of the light blocking member 303B.

1-2. Working Example 2

[0172]FIG. 20 is a sectional view showing a light blocking layout
structure according to a working example 2.

[0173]In the light blocking layout structure according to the working
example 1, the light blocking member 303G is provided below the metal
wiring layer 301G. In contrast, in the light blocking layout structure
according to the present working example 2, the light blocking member
303G is provided below an auxiliary line 306 provided in the same layer
as that of the metal wiring layer 301G between the pixels. Here, the
auxiliary line 306 is normally disposed so as to surround each of the
pixels in order to supply the cathode potential Vcath to the cathode
electrode of the organic EL element 21.

[0174]For the light blocking member 303G, for example, a material same as
that of the auxiliary line 306, that is, a metal material having high
reflectivity such as aluminum, is used. While the light blocking member
303B for the B pixel is shown in FIG. 20, also the insulating films
303G-1 to 303G-4 in the modification 1 or the modification 2 to the
working example 1 may be embedded at a lower end portion thereof in the
gate insulating film 304. This similarly applies also to the B and R
pixels 20B ad 20R.

[0175]In this manner, also where the light blocking member 303B is
provided below the auxiliary line 306, it is possible to block blue light
emitted from the B pixel 20B from being inputted to the R and G pixels
20R and 20B similarly as in the working example 1. In addition, the
location below the auxiliary line 306 is outside the pixels, and there is
an advantage that it is easier to produce the light blocking member 303G
in regard to the space than where the auxiliary line 306 is provided
below the metal wiring layer 301G and a large aperture can be formed.

2. Modifications

[0176]In the embodiment described above, the driving circuit for the
organic EL element 21 basically has a 2-Tr configuration which includes
two transistors (Tr) including the driving transistor 22 and the writing
transistor 23, the present invention is not limited to the 2-Tr
configuration. In particular, the driving circuit can have various pixel
configurations such as, a pixel configuration which includes a transistor
for controlling light emission/no-light emission of the organic EL
element 21 in addition to the two transistors and another pixel
configuration which additionally includes a switching transistor for
selectively writing the reference voltage Vofs into the gate electrode of
the driving transistor 22.

[0177]Further, while, in the embodiment described above, the present
invention is applied to an organic EL display apparatus which uses an
organic EL element as an electro-optical element of a pixel, the present
invention is not limited to this application. In particular, the present
invention can be applied to various display apparatus which use an
electro-optical element, that is, a light emitting element, of the
current driven type whose emitted light luminance varies in response to
the value of current flowing to the element such as an inorganic EL
element, an LED element or a semiconductor laser element.

3. Applications

[0178]The display apparatus according to an embodiment of the present
invention described above can be applied to a display apparatus for
electronic apparatus in various fields wherein an image signal inputted
to the electronic apparatus or an image signal produced in the electronic
apparatus is displayed as an image.

[0179]With the display apparatus according to an embodiment of the present
invention, it is possible to suppress a characteristic shift caused by an
influence of irradiation of light having high energy upon the channel of
a pixel transistor thereby to suppress reduction of current to flow to
the organic EL element and appearance of a fault in picture quality such
as stripes and luminance unevenness. Accordingly, by using the display
apparatus according to an embodiment of the present invention as a
display apparatus of electronic apparatus in various fields, enhancement
of the display quality of the display apparatus of the electronic
apparatus can be anticipated.

[0180]The display apparatus according an embodiment of the present
invention may be of the module type having an enclosed configuration. The
display apparatus of the module type corresponds, for example, to a
display module wherein an opposing element of transparent glass or the
like is adhered to a display array section. On the transparent opposing
portion, a color filter, a protective film and so forth as well as the
light blocking film described hereinabove may be provided. It is to be
noted that the display module may include a circuit section for inputting
and outputting signals and so forth from the outside to the pixel array
section and vice versa, a flexible printed circuit board (FPC) and so
forth.

[0181]In the following, particular examples of an electronic apparatus to
which an embodiment of the present invention is applied are described. In
particular, the present invention can be applied to such various
electronic apparatus as shown in FIGS. 21 to 25A to 25G, for example, to
a digital camera, a notebook type personal computer, a portable terminal
apparatus such as a portable telephone set and a video camera.

[0182]FIG. 21 shows an appearance of a television set to which an
embodiment of the present invention is applied. Referring to FIG. 21, the
television set shown includes a front panel 102 and an image display
screen section 101 formed from a filter glass plate 103 and so forth and
is produced using the display apparatus according to the embodiment of
the present invention as the image display screen section 101.

[0183]FIGS. 22A and 22B show an appearance of a digital camera to which an
embodiment of the present invention is applied. Referring to FIGS. 22A
and 22B, the digital camera shown includes a flash light emitting section
111, a display section 112, a menu switch 113, a shutter button 114 and
so forth. The digital camera is produced using the display apparatus
according to the embodiment of the present invention as the display
section 112.

[0184]FIG. 23 shows an appearance of a notebook type personal computer to
which an embodiment of the present invention is applied. Referring to
FIG. 23, the notebook type personal computer shown includes a body 121,
and a keyboard 122 for being operated in order to input characters and so
forth, a display section 123 for displaying an image and so forth
provided on the body 121. The notebook type personal computer is produced
using the display apparatus according to the embodiment of the present
invention as the display section 123.

[0185]FIG. 24 shows an appearance of a video camera to which an embodiment
of the present invention is applied. Referring to FIG. 24, the video
camera shown includes a body section 131, and a lens 132 for picking up
an image of an image pickup object, a start/stop switch 133 for image
pickup, a display section 134 and so forth provided on a face of the body
section 131 which is directed forwardly. The video camera is produced
using the display apparatus according to the embodiment of the present
invention as the display section 134.

[0186]FIGS. 25A to 25G show an appearance of a portable terminal
apparatus, for example, a portable telephone set, to which an embodiment
of the present invention is applied. Referring to FIGS. 25A to 25G, the
portable telephone set includes an upper side housing 141, a lower side
housing 142, a connection section 143 in the form of a hinge section, a
display section 144, a sub display section 145, a picture light 146, a
camera 147 and so forth. The portable telephone set is produced using the
display apparatus of the embodiment of the present invention as the
display section 144 or the sub display section 145.

[0187]The present application contains subject matter related to that
disclosed in Japanese Priority Patent Application JP 2008-325072 filed in
the Japan Patent Office on Dec. 22, 2008, the entire content of which is
hereby incorporated by reference.

[0188]It should be understood by those skilled in the art that various
modifications, combinations, sub-combinations and alterations may occur
depending on design requirements and other factor in so far as they are
within the scope of the appended claims or the equivalents thereof.